Numerous patents and publications teach that boron can be extracted from aqueous solutions by anion exchange techniques. Anion exchange extractants for extracting boron from aqueous solutions are commercially available.
The treatment of geothermal brines to recover boron is complicated by the complex composition of the brine, including relatively low concentrations of boron, and by the temperature of the brine. These brines contain anions having Lewis acidities both greater and less than boric acid, which can make selective boron extraction difficult, if not impossible.
Geothermal brines are used as a heat source for the generation of electric power when brought to the earth surface and allowed to flash into steam which is passed through expansion turbines. However, even after passing through expansion turbines, the temperature of the brine is generally in excess of 90.degree. C., e.g. 95.degree. C. The relatively high temperature of these brines and the large volumes of brine involved preclude the use of liquid extractants for the recovery of boron therefrom. The high temperatures of the brines significantly increase the loss of organic solvents through evaporation and, possibly, through increased solubility in the brines. The large volumes of brine which must be treated increase losses of organic solvents through the mass action mechanism thereby increasing operating costs and environmental risks.
In the late seventies and early eighties the Department of Energy sponsored a number of R and D programs to investigate the generation of geothermal power from Imperial Valley resources in California. One of the programs included was a budget of $20 million or more to investigate the potential of producing power from a very deep brine field in the Southern section of the valley near Brawley which is called the Imperial Field. This resulted in the drilling of four wells in excess of 15,000 feet that were tested for an extended period of time. It appears that a CU-1 joint venture (a group interested in the production of electrical power from geothermal resources) responsible for the test program decided against the development of these deep resources because of various technical problems. One of the problems was associated with the formation of a heavy scale on the inside of the wellbore, which resulted in a premature decrease in the flow of brine. Although experience had been gained with solving scaling problems that occurred in shallower wells in the Northern section of the Valley, this scale had a different characteristic. The scaling was the result of the brine being supersaturated with lead and silver, with the CU-1 wellbore scales running about 80% lead and 10% silver.
Brine samples from these four wells showed zinc values that were 2 to 4 times higher than the ones experienced with existing geothermal brines from the Northern section of the Imperial valley. No further work was done to solve the scaling problem or the problem of metal recovery and the programs were discontinued.
The Imperial Valley of California presently supports about half a dozen geothermal power plants with a total installed generating capacity for 250 MW. At present, four more plants are under construction which will increase this capacity by an additional 150 MW.
An analytical comparison of flashed brines from the Imperial Field and the Salton Sea Field in parts per million is given below:
______________________________________ Element, ppm Imperial Field Salton Sea Field ______________________________________ Gold No 0.2 Silver 2.6 0.5 Zinc 1500 500 Lead 650 110 Lithium 250 280 Strontium 1500 620 Manganese 1000 1200 Platinum NA 0.06 Sodium 50,000 57,000 Calcium 18,000 26,000 Potassium 10,000 15,000 Iron 3,200 1,800 Barium 2,000 600 Boron 220 360 Chloride 131,000 160,000 ______________________________________ As will be noted, the chemical composition of brines from both fields is essentially the same, except for zinc, lead and silver.
The Salton Sea Field exhibits high salinity at a relatively shallow depth and contains approximately 28% dissolved solids; whereas, the Imperial Field, compared to the Salton Sea Field, has higher salinity and has dissolved solids of approximately by weight 32% at greater depths.
Various methods have been proposed for treating geothermal brines. Particular reference is made to U.S. Pat. No. 4,624,704 by J. J. Byeseda which is directed to the selective recovery of zinc from metal containing brine, the disclosure of which is incorporated herein by reference.
As stated in this patent, geothermal brines are of particular interest in that they provide a source of power by virtue of the fact that hot geothermal pools are stored under great pressure beneath the earth surface and which when released to atmospheric pressure provide flash-steam for running a power plant. In addition, the geothermal brine contains metal values as stated hereinbefore, especially lead, silver and zinc, zinc being predominant of the three elements.
Up until the present time, no commercially viable process for recovering boron from geothermal brines has been available. Geothermal brines can be treated to recover other valuable minerals as disclosed in U.S. patent application Ser. No. 763,446, filed Sep. 19, 1991 and assigned to the present assignee, which application is incorporated herein by reference.